1. Field of the Invention
The present invention relates to a printing apparatus and a printing method that perform printing by using a print head having a plurality of print elements arranged in columns, and more particularly to a printing apparatus and a printing method suitably applied to cases where a so-called full multi-type ink jet print head with a large number of ink nozzles arrayed over a relatively long range is used.
2. Description of the Related Art
Printing apparatus used in printers and copying machines and printing apparatus used as output devices for composite electronic devices including computers and word processors and for workstations are constructed to form images (including characters and symbols) on a print medium such as a paper and a thin plastic sheet according to print information (printing information). These printing apparatus may be classified into an ink jet system, a wire dot system, a thermal system and a laser beam system according to the printing method employed.
The printing apparatus may also be grouped into a serial type and a line type in terms of a printing action. In the serial type, a print head as a printing means is scanned over a print medium in a main scan direction that crosses a subscan direction in which the print medium is transported. That is, an entire area of the print medium is printed by repetitively feeding the print medium a predetermined distance each time the print head completes one main scan printing.
In the line type, printing is done by moving only the print medium in the subscan direction without moving the print head in the main scan direction. This type of printing apparatus prints an entire area of the print medium by first setting the print medium at a predetermined position and then feeding the print medium in the subscan direction while at the same time printing lines of print data, one line at a time, on the print medium.
Of these types of printing apparatus, the ink jet printing apparatus performs printing by ejecting ink from nozzles of a printing means (print head) onto a print medium. The ink jet printing apparatus has many advantages, such as an ease with which the print head can be reduced in size, an ability to print a high resolution image at high speed, an ability to print on plain paper without having to apply a special treatment to it, a low running cost, low noise realized by a non-impact printing, and an ease with which a color image can be printed using multiple color inks. In a line type ink jet printing apparatus which uses a full multi-type print head having a large number of print elements arrayed in a width direction of the print medium (in a direction crossing, normally at right angles to, the print medium transport direction), a further increase in the printing speed can be achieved. Further, the line type printing apparatus is attracting attention because of a possibility of it being used as an on-demand type printer for which there is a growing demand in the market. The print elements are arranged to span a full width of a print area of the print medium and eject ink from the nozzles.
Unlike conventional printing of newspapers and magazines which is required to print millions of copies, the on-demand printing is not required to have such a high speed printing capability as 100,000 sheets per hour but is called upon to reduce labor. The full multi-type line printer, though its printing speed is far slower than that of a conventional printing press such as offset printer, can save labor because there is no need to make a printing plate and is thus optimal for on-demand printing.
The full multi-type line printer for use in such on-demand printing is required to produce printed mediums of A3 size at a rate of 30 pages or more per minute at a resolution of 600×600 dpi (dots per inch) for mono-color text documents and at a high resolution of 1200×1200 dpi or higher for full color photographs and the like.
In a print head used on the abovementioned full multi-type printer, however, forming all ink jet print elements arranged to span the full width of a print area of the print medium, particularly nozzles constituting a part of the ink jet print elements, without any defects is not an easy task. For example, in a full multi-type printer that produces photographic quality outputs on large-sized paper, such as those used in offices, to print a 1200-dpi outputs on A3-size paper requires forming about 14,000 nozzles in a full multi-type print head (with a print width of about 280 mm). It is difficult to manufacture all the ink jet print elements corresponding to such a large number of nozzles without any defects. Even if such a print head can be manufactured, it is easily conceivable that a yield may be low and a manufacturing cost prohibitively high.
To deal with this problem a so-called combined head is proposed as the full multi-type print head for use in the line type ink jet printing apparatus. The combined head is made by joining end-to-end a plurality of relatively inexpensive, short chip-type print heads, such as those used in a serial type, in a longitudinal direction of nozzle array or column with a high precision to increase the length of the print head.
Because of this construction, however, the combined head has a problem that those portions of a printed result corresponding to the print elements situated at joints between the chips tend to be degraded. In more concrete terms, any deviation in the arrangement of chips may result in pitches between adjoining nozzles (simply referred to as nozzle pitches) at the joints failing to agree with those of other nozzles. Should this take place, density variations that look like stripes or bands (joint stripes) appear on a printed image at locations corresponding to the joints between the chips.
To address these joint stripes some countermeasures have been proposed.
For example, a method is available which reduces the nozzle pitch deviations by using a particular chip arrangement technique or device that aligns chips at the joints with high precision. Another chip arrangement method involves staggering the chips in a print medium transport direction and overlapping the adjoining chips by a predetermined number of nozzles at chip ends. In this case, during printing, the overlapping nozzles of both of the adjoining chips are made to eject ink to make the joint stripes less conspicuous. Still another method is to change the volume of ink droplets ejected from the nozzles at the joints to make the joint stripes less conspicuous.
These methods, however, cannot reduce the joint stripes to an acceptable level when producing a photographic quality print.